Liquid crystal display devices have conventionally been used for an image displaying part in various display apparatuses such as a projection display (projector), various portable electric devices, and various information processing terminals, and the like. The liquid crystal display device is called a liquid crystal panel or a liquid crystal cell. The kinds of the liquid crystal display device are broadly classified into a transmission type and a reflective type. The liquid crystal display devices have a structure in which liquid crystal is enclosed between a pixel electrode substrate and an opposite substrate facing the pixel electrode substrate. In the transmission liquid crystal display device, a transparent electrode made of ITO (Indium Tin Oxide) is located on each of the pixel electrode substrate and the opposite substrate.
On the other hand, as fineness, miniaturization and brightness of the liquid crystal projectors are improved in recent years, the reflective liquid crystal display device is remarkably expected to provide miniaturization, high fineness, and high efficiency of light-using as a display device, and actually, it is in practical use. In the reflective liquid crystal display device, a transparent electrode made of ITO is located facing the opposite substrate, and a reflective pixel electrode (hereinafter referred to just as “reflective electrode”) is located facing the pixel electrode substrate. The reflective liquid crystal display device used for the liquid crystal projector is generally an active type, and an object having C-MOS (Complementary-Metal Oxide Semiconductor) type semiconductor switching circuit formed on a silicon substrate is used as a pixel electrode substrate. The reflective electrode is located on the silicon driving-devices substrate. The reflective electrode has functions of reflecting light which enters from the side facing the opposite substrate, and of applying voltages to the liquid crystal. As a material of the reflective electrode, metallic materials, which are generally used in LSI (Large Scale Integrated) processes and contain aluminum (Al) as a principal component, are used.
In the reflective liquid crystal display device, voltages are applied to the liquid crystal by the transparent electrode and the pixel electrode which are located respectively on the substrates. At this time, the liquid crystal changes its optical properties depending on a potential difference between the opposing electrodes and modulates incident light. The optical modulation permits gradation display, and the modulated light is used for displaying images.
On the other hand, in the liquid crystal display device, a driving method of reversing polarity to plus or minus for every predetermined period, and applying voltages between the electrodes are generally used in order to prevent ions existing in the liquid crystal from causing burn-in on either of the substrates which is operated. FIG. 18 is a schematic diagram showing a driving voltage using the driving method. As expressed by solid line shown in the diagram, if absolute value of each polar voltage applied between the opposing electrodes is the same at V1, it is supposed that there is no difference between actual voltages applied to the liquid crystal, and that the phenomena such as the burn-in as described above does not occur. However, in fact, particularly in the reflective liquid crystal display device, there is a difference between the actual voltages of plus and minus applied to the liquid crystal. This results from different electrode materials which are used respectively for both substrates in the reflective liquid crystal display device.
That is, in the reflective liquid crystal display device, ITO is generally used as the transparent electrode as described above, and an aluminum metal film containing slight copper or the like is used as the opposed pixel electrode. In this case, both electrodes, which are respectively made of ITO and aluminum, have different standard electrode potentials from each other, so a battery effect is generated in the device using these electrodes which are made of different metals. The standard electrode potential of aluminum is −1.66 V, and combination of the aluminum electrode with the ITO electrode generates a quite high battery effect between these electrodes.
For this reason, even if voltages with both polarities having the same absolute value as expressed by a solid line in FIG. 18 is applied from the outside, the battery effect generates electromotive force, and asymmetric voltages are applied to the liquid crystal. Consequently, the reflectance of the device depends on the polarities of the applied voltages, which causes flicker and stores an internal voltage in the device, resulting in problems such as the burn-in. If an aluminum electrode is used instead of the ITO transparent electrode, that is, if both of the opposing electrodes are similarly made of aluminum, the battery effect is cancelled out, and the above asymmetry does not occur. However, this is not practical because no light passes through the device. Moreover, it is obvious that the problem of the asymmetry does not occur because the electrodes are the same kind in conventional transmission liquid crystal devices which have the opposing electrodes made of ITO. Therefore, the asymmetry is an essential problem in the reflective liquid crystal device.
In order to eliminate the asymmetry in the reflectance of the reflective liquid crystal display device, the driving voltages with both polarities having different absolute values, which are obtained by adding DC offset voltage ΔV to the driving voltage, should be applied for as expressed by a dashed line in FIG. 18. For example, when aluminum is used as a reflective electrode material and ITO is used for the opposite transparent electrode, an effective voltage difference between both the polarities being applied to the liquid crystal is 1 V or more, and the difference is applied as the offset voltage ΔV. However, if the value of the offset voltage ΔV is too high, the asymmetry cannot be eliminated completely, and furthermore, the offset voltage ΔV is gradually changed from an initial setting value during long term driving, and the internal voltage in the device is stored as a result, so the burn-in occurs. This decreases reliability during the long term driving. Moreover, in order to apply the offset voltage ΔV, it is necessary to prepare a circuit for applying it, and an electric circuit system thereof becomes complicated. Therefore, in the reflective liquid crystal display device, the battery effect is not preferable, essentially.
On the other hand, Japanese Patent Laid Open Nos. 9-244068 and 10-54995 teach that metals with a standard electrode potential which is lower enough than that of aluminum, for example, tungsten, (W), titanium (Ti), and titanium nitride (TiN), are used as the reflective electrode material in order to alleviate the above problem of the voltage difference, avoid the battery effect, and reduce the offset voltage.
However, using tungsten, titanium, and titanium nitride as the reflective electrode material, gives insufficient reflectance compared with generally used aluminum, and thus they are unsuitable electrode materials. Therefore, technology developments for achieving reduction of the offset voltage without damaging a light reflective function of the reflective electrode are desired.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a reflective liquid crystal display device and a liquid crystal display which can prevent the battery effect which is a factor of the asymmetry in liquid crystal responses, reduce the offset voltage applied to the driving voltage, and secure high reliability even during the long term driving.